A literature review of demonstrations for photo-electrochemical hydrogen production suggests that interesting solar-to-fuel efficiencies can be obtained by integrating expensive III-V and rare earth elements as photoabsorbing and catalytic materials. Aiming just at high solar-to-fuel efficiency at the expense of low electrochemical current densities is of little use for practical device implementations, particularly for these expensive and high performing materials. The challenge for these photo-electrochemical device demonstrations is to achieve higher operating current densities while maintaining high efficiencies. A close thermal and electronic integration of the photoactive and electrochemical component allows to achieve this goal. Here, we report on the successful experimental demonstration of an integrated photo-electrochemical device employing concentrated solar irradiation and smart thermal management strategies in order to achieve a solar-to-hydrogen efficiency as high as 17.2% along with an electrochemical current density of 900 mA/cm_EC² and a photovoltaic current density of 6.04 A/cm_PV².

The implemented device (figure 1.a) incorporates a solid ionic conductor as the separator between anode and cathode. The ionic conductor is a nafion 115 membrane which is coated on one side with IrO₂ and on the other side with Pt black. The membrane electrode assembly is sandwiched between a Pt coated Ti mesh (a gas diffusion layer) and the custom made Ti bipolar flow plates. The anodic Ti plate is designed with a stage for the direct integration of the photovoltaic (PV) component. The device additionally incorporates a steel plate with a glass window which acts as a channel for the reactant (water), which is guided directly on top of the PV part. This removes any excess heat from the PV, pre-heats the reactant and directly feeds it to the EC part, allowing to benefit the device performance by reducing the PV’s temperature while increasing the electrocatalysis temperature. The device has been tested in our high flux solar simulator facitlity¹ (shown in Fig. 1b), which simulates concentrated irradiation homogeneously over an area as large as 20mm by 20 mm. The photoactive area is 4cm² and the electrocatalytically active area is 25 cm².

The device has been tested in integrated mode for varying concentrations, ranging between 117 and 474 kW/m². At all these concentrations, a steady state performance was recorded for about 10 minutes and the output hydrogen gas flow was measured, using a combination of water traps (to dry the output stream) and precision flowmeters. The measured, dry hydrogen flow rates correspond to a solar-to-hydrogen efficiency of 17.2%, one of the highest efficiencies ever reported, at an electrochemical current densities up to 900 mA/cm_EC². Additionally, we have demonstrated one of the highest output power devices (in terms of hydrogen equivalent) for photo-electrochemical production of hydrogen. The dynamic response and stability testing (for ~2 hours) has shown that the implemented device can perform efficiently under varying conditions and for long periods of time.

The implemented device opens a new pathway towards scalable and large-scale deployment of photo-electrochemical water splitting. The prototype validates the results of our previously developed 2D multi-physics model ^2,3 and demonstrates an efficient and cost effective way of solar hydrogen processing.

References:

1 G. Levêque, R. Bader, W. Lipiński and S. Haussener, Opt. Express, 2016, 24, A1360.

2 S. Tembhurne and S. Haussener, J. Electrochem. Soc., 2016, 163, 999–1007.

3 S. Tembhurne and S. Haussener, J. Electrochem. Soc., 2016, 163, 988–998.